In information theory, an observer is any system which receives information from an object.

Observer can have the following meanings:

• A person that is observing, its role in observational sciences and physical reference frames: see observation
• A delegate in an organization that is sent to observe and report on the proceedings of an assembly or a meeting but that can not vote or otherwise participate.
• An aircrew member of a (military) aircraft responsible for searching (usually visually) for intelligence, opposing forces and survivors of interest to the mission
• A state observer used in control applications. In most control systems not all state variables of the plant can be measured. If it is an observable plant the state can be estimated with the help of a state observer. The estimated state is often used for full state feedback.
• A UK Sunday newspaper: see The Observer
• The Texas Observer, an American political magazine
• A Sri Lanka Sunday and Evening newspaper: See List of newspapers in Sri Lanka
• A design pattern used in computer programming: see Observer pattern
• "The Observer," a track from the Flaming Lips' 1999 album The Soft Bulletin
• A flying unit in the game StarCraft: see Observer (StarCraft)

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